A data over cable service interface specification (DOCSIS) 3.1 standard supports various data transmission rates through non-square quadrature amplitude modulation (QAM) having odd bits and square QAM having even bits.
A low density parity check (LDPC) code is combined thereto, and as a result, users can acquire a channel capacity which approximates a Shannon limit. A soft decision value is used for LDPC code demodulation and a representative method for calculating the soft decision value is a log likelihood ratio (LLR).
The square QAM can independently separate a bit constituting in-phase and a bit constituting quadrature and a log likelihood ratio calculation equation for the bit constituting the in-phase may be used as a log likelihood ratio calculation equation for the bit constituting the quadrature.
Contrary to this, since it is impossible for the non-square QAM to independently separate the bit constituting the in-phase and the bit constituting the quadrature, in particular, a more complicated calculation process needs to be performed in order to calculate the log likelihood ratio for the high-degree non-square QAM.